Ion implantation is a physical process that is employed in semiconductor device fabrication to selectively implant dopant into semiconductor and/or wafer material. Thus, the act of implanting does not rely on a chemical interaction between a dopant and semiconductor material. For ion implantation, dopant atoms/molecules are ionized, accelerated, formed into a beam, analyzed, and swept across a wafer, or the wafer is swept through the beam. The dopant ions physically bombard the wafer, enter the surface and come to rest below the surface, at a depth related to their energy.
Referring to FIG. 1 ion implanters or ion implantation systems typically include three sections or subsystems: (i) an ion source chamber 102 containing an ion source for outputting an ion beam, (ii) a beamline assembly 110 including a mass analysis magnet for mass resolving the ion beam, and (iii) a process chamber 112 which contains a target location that receives the ion beam from the beam line assembly, such as a semiconductor wafer 114 or other substrate to be implanted by the ion beam. The continuing trend toward smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow implants. Source/drain junctions in semiconductor devices, for example, require such a high current, low energy application.
Ion sources in ion implanters typically generate an ion beam by ionizing within the source chamber 102 a source gas, a component of which is a desired dopant element, and extracting the ionized source gas in the form of an ion beam. The ion source may take the form of an inductively heated cathode (IHC), typically utilized in high current ion implantation equipment.
Examples of desired dopant elements of which the source gas is comprised include boron (B), germanium (Ge), phosphorus (P), or silicon (Si). The source gas may be, for example, a fluorine-containing gas, such as boron trifluoride (BF3), germanium tetrafluoride (GeF4), phosphorous trifluoride (PF3), or silicon tetrafluoride (SiF4), amongst others.
When the ion source is operated with a molecular source gas, species in addition to the desired species for implantation are often produced, resulting in ion source failure due to the accumulation or corrosive properties of these species generated during disassociation/ionization of the source gases. Some of these species may have very low vapor pressures, and as a result condense on the interior surfaces of the source. These solid deposits may interfere with ion source operation over time, for example by changing the electrical characteristics of the walls or partially blocking the ion source electrode aperture, thereby reducing the available ion current.
The decreased lifetime of ion sources used in germanium and boron ion implantation can be ascribed to the generation of free fluorine radicals during dissociation of germanium tetrafluoride (GeF4) and boron trifluoride (BF3) source gas. These fluorine radicals react with the refractory metals, such as tungsten and molybdenum, commonly used to construct the ion source chambers in order to provide structural integrity at elevated operating temperatures of 700° C. or more. The tungsten hexafluoride (WF6) or molybdenum fluoride (MoF6) molecules decompose on hot surfaces and condense at the cathode surface in what is known as the halogen cycle. The WF6 and MoF6 molecules generate six additional free fluorine radicals each, thereby accelerating WF6 and MoF6 formation. These molecules do not decompose spontaneously on chamber surfaces without the presence of a reducer to strip the fluorine atoms. The tungsten and molybdenum molecules accumulate on cathode surfaces, increasing cathode size and resulting in the degradation of electron emissions from the surface of the cathode.
Additionally, excess free fluorine radicals in the ion source chamber 102 can result in etching of the chamber housing material and internal components. Fragile columnar structures build up and break off, causing discharges by either bridging the cathode or repeller to ground or being ejected into the extraction/extraction suppression high voltage field causing a discharge This material can then be transported down the beamline to the wafer. It has been shown that material or debris that is generated inside the ion source chamber may be extracted and transported to the substrate. These particulates have a direct effect on semiconductor device yield.
One method of removing these deposits is the time consuming process of removing the ion source from the system and physically cleaning the source or running a species of gas to sputter clean the arc chamber at some predetermined interval. This process is not highly effective, and in either case tool productivity is severely impacted. Another method is to clean the source in situ by flowing a highly reactive gas through the source, where the gas species is chosen such that the flourine gas radicals are captured, and pumped away before they can attack the internal and external ion source components.